I have had this problem for quite a while, when driving the vehicle after it is at operating temp the idle will begin to pulse or hunt from about 1100 to 1800 and if it stops pulsing it idles anywhere from 1500 to 2500 steady. I have read many posts trying to discover what it may be so here is where i have been and what i have done to try to fix this, checked all vacuum lines for cracks and routing, replaced if necessary, replaced o2 sensor, replaced throttle body with spare parts one but not new, cant afford $700 for just a throttle body, removed and blocked lines from cold idle air sensor, fast idle air sensor and thermal valve. I had this problem before with my 88 but unfortunately the dear killed my car and not the problem, I would like to get this taken care of as i know my 88 got gradually worse, it got to the point it started idling very fast from 1200 to 3500 and rocked the entire car and i know that cant be good on a motor. please any help will b much appreciated, this is my winter vehicle and offroad toy and will probably be my every day driver for the next year until i finish rebuilding the motor for my on road toy so need it dependable

If the ecm gets a signal from the throttle position sensor that the throttle plates are fully closed the ecm puts the engine in "idle mode" which set the timing at 12* steady and ignores the signal from the o2 sensor.

Now with that said, if the throttle plates are fully closed OR the throttle position sensor makes "idle" connection before the throttle plates close fully the ecm will check the rpm speed. If the rpm's are higher then (I don't know exactly) but somewhere over 1200 rpms, then the ecm will cut the fuel injectors momentarily to keep the engine from idling too high or red lining.

So your problem exist somewhere between the throttle body and the mass air flow meter or the coolant temp sensor .

Could be that your idle is set too high.
Could be the thermo-valve on top of the thermostat,(I know you checked already)
Could be the aux air control valve under the intake.
Could be the fast idle solenoid on the passenger fender
Could be the coolant temp sensor under the thermostat housing
And last but not least, a malfunctioning throttle position sensor.

Good luck

1988 to 98 Isuzoo Trooper II. rebuilt with american pride! Built and tested now parked and resting until the next adventure!

thank you Ben that gives me a few more leads to work on im pretty sure the aux air control valve or the fast idle solenoid arent the issue as ive had them both completely unhooked and same problem, the fast idle soleniod doesnt help though, it just makes it worse when on. as in my signature this motor has been thru a couple overheatings due to burst hoses and faulty temp gauge, but my gauge is fixed now after a really bad mistake on my part listening to the parts house guy who thought he was a mechanic, i took his advice and pulled the temp sensor for the ecm out cuz he said it was the sensor for the gauge and replaced it with a manual gauge unit, then found the planet and figured out why the car wouldnt run when hot, put the old temp sender for ecm in and it ran again, then found the sensor for temp gauge on side was just unplugged. anyway would overheating cause the ecm temp sensor to go bad? i also have a very small exhaust leak where the manifold meets the downpipe that im going to try another new gasket on but afraid it may be a crack or burnt hole from leaking so long, which i have been told could be the problem. also i am not running an isuzu thermostat, i know shame shame on me, but the one i have seems to work just maybe running a little hot, not sure, its supposed to be a 180 but could be off being a cheap brand, would trying a cooler thermostat be of any advantage since i have a 160 i havent tried yet? also some quick-fix-it advice site i came accross mentioned putting a new catalytic converter on would solve it, i doubted it since they were trying to sell me one, any thruth to that at all? also what is the process for resetting the ecm? i had some CEL issues when i pulled out the ecm temp sensor but not anymore since fixing do i still need to reset it?

btw how do i check the TPS to see if its bad? and if its not what do i need to do to make sure its adjusted right?

This is the reason the zoo forum is so cool, somebody somewhere knows the answer, or how to find the answer to most Isuzu questions.

On the 2.6L with 5spd, the TPS is very simple. It has two internal switches and three connecting wires. The TPS is connected to the ECM and sends it three different signals.
1) Idle: with the throttle valves closed.
2) Off-idle: with throttle valves opened.
3) WOT: wide-open-throttle with valves completely open.

Here are some links to FSM pages that cover the adjustments: these were provided by isuzufool.

Here is where the TPS is located. It is mounted on the throttle valve shaft and that shaft opens and closes the switches as it turns. A: is the hold-down screw. Loose this to adjust the TPS. B: is the electrical connector on the TPS.

I had this same problem after my head gasket replacement adventure. 'Twas the idle speed. It was set too high. Another thing to check, if you have the throttle body with the two throttle plates in it, make sure the lower plate is closing all the way. This was a problem that drove me nuts. If you look at the throttle linkage where the cable attaches, you'll notice that there is a linkage to open the other throttle plate, below the one the cable pulls on. There is a bit of slack in the linkage, when your truck is idling too fast, see if moving the linkage for the lower throttle plate makes a difference.

STUCK: A condition reached when you head out in the deep snow, with a vehicle other than an Isuzu.

This thread seems like it could be related to my situation. I don't have a Trooper, I have an Amigo, but I do have the 2.6 and 5spd.

I too had a head gasket/head repair recently, and since then, my idle surges. It doesn't seem to matter much if the engine is hot or cold, but it fluctuates a few hundred RPMs about every 2 seconds or so.

I see by MarKB's pages, the throttle plate should be closed at idle. I think mine is slightly open to keep the idle up. I adjusted the cable tension with the cable mounting nut near the throttle linkage. If I ease the tension, the idle is rough and has been known to stall the motor. What's another way to keep the idle up? The throttle stop screw is factory set and painted to stay put.